1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer and the like.
2. Related Background Art
In conventional electrophotographic image forming apparatuses using an electrophotographic image forming process, a process cartridge system in which an electrophotographic photosensitive member and process means acting on the electrophotographic photosensitive member are integrally incorporated as a cartridge unit, which can detachably be mounted to a main body of the image forming apparatus, has been used. According to such a process-cartridge system, since maintenance of the apparatus can be performed by an operator himself without any expert, operability can be enhanced considerably. Thus, the process-cartridge system has widely been used with the electrophotographic image forming apparatus.
In a representative process cartridge, at least one of electrifying means, cleaning means and developing means, and an electrophotographic photosensitive member are integrally incorporated as a cartridge unit that can detachably be mounted to a main body of the image forming apparatus.
In such a process cartridge, the developing means comprises a developing member as developing means, and a developer containing portion containing developer (referred to as xe2x80x9ctonerxe2x80x9d hereinafter).
When the process cartridge is used for a long time, the photosensitive drum is worn to generate a poor image, plagued by fog. To cope with this, conventionally, as means for notifying the user of the end of the service life of the photosensitive drum, there has been developed a technique in which the total number of revolutions of the photosensitive drum or the total rotating time of the photosensitive drum is detected, and, when the detected value exceeds a predetermined value, it is judged that the end of the service life of the photosensitive drum is reached.
Further, as improvement, there has been proposed a technique in which the total applying time of the AC current applied to the electrifying means is also considered, as well as the total number of revolutions of the photosensitive drum or the total rotating time of the photosensitive drum.
Further, there has been proposed a technique in which the number of revolutions of the photosensitive drum or the rotating time of the photosensitive drum is stored in an IC memory (for example, an EEPROM) as storing means attached to a drum unit having the photosensitive drum or to the process cartridge of an integral type. As the IC memory, Japanese Patent Application Laid-Open No. 10-221938 also discloses a memory of non-contact type.
As the electrophotographic image forming apparatus utilizing the process cartridge system, an example of a conventional laser beam printer is shown in FIG. 9.
The laser beam printer includes a photosensitive drum 101, an exposing apparatus 102, a developing apparatus 103, a transferring member 104, a cleaning apparatus 105, an electrifying member 106, a fixing apparatus 107, a sheet feeding cassette B containing transfer materials, and a sheet feeding apparatus 108. The photosensitive drum 101, the developing apparatus 103, the cleaning apparatus 105 and the electrifying member 106 constitute a process cartridge C.
By projecting a laser beam L corresponding to image information from the exposing apparatus 102 onto a surface of the photosensitive drum 101 electrified with a desired potential by the electrifying member 106 to remove electricity, an electrostatic latent image is formed on the photosensitive drum 101.
The developing apparatus 103 includes a developing container 133 as a developer containing portion containing toner T, a developing sleeve 131 as a developing member, and a doctor blade 132. The toner T is supplied from the developing sleeve 131 to the electrostatic latent image on the photosensitive drum 101, thereby forming a toner image. Thereafter, the toner image on the photosensitive drum 101 is transferred onto a surface of the transfer material P by means of the transferring member 104. The unfixed toner image on the transfer material P is permanently fixed to the transfer material P with heat and pressure by means of the fixing apparatus 107. Then, the transfer material is discharged out of the printer.
On the other hand, after the transferring operation, residual matter, such as toner and paper powder, remaining on the photosensitive drum 101 is removed by the cleaning apparatus 105.
Further, a toner-remaining-amount detecting rod 111 is disposed in parallel with the developing sleeve 131 so that the remaining amount of toner is detected by detecting the electrostatic capacity between the developing sleeve 131 and the rod.
In the image forming apparatus using the process-cartridge system, as the process cartridge is being continuously used from a new cartridge, an image property is changed.
The image property is changed because of a fluctuation in the sensitivity of the photosensitive drum and a fluctuation in the toner electrifying condition. The fluctuation in the sensitivity of the photosensitive drum is caused by repeatedly performing electrification, exposure, and electricity removal. Under the same image forming condition, if the sensitivity of the photosensitive drum tends to be increased, by repeating the image forming operations, when the device tries to produce identical lines, the produced lines will become gradually fat. Conversely, if the sensitivity of the photosensitive drum tends to be decreased, when the device tries to produce identical lines, the produced lines will become gradually thin.
Further, regarding the toner electrifying condition, the electrifying amount is gradually increased by repeating the image forming operations from a condition that the toner is new. If the amount of toner electrified within a proper range is increased, when the device tries to produce identical lines, the produced lines will become gradually fat. Conversely, if the electrifying amount of toner is increased to exceed the proper range, when the device tries to produce identical lines, the produced lines will become gradually thin. Such a phenomenon is apt to occur particularly in a low humidity environment.
In consideration of such a fluctuating property, generally, a saturated level of the fluctuating property is set as a proper range of image. Thus, regarding the whole use range of the process cartridge, it is set so that the optimum image forming conditions are a maximum.
FIG. 10 shows a fluctuation in a line width as a function of an accumulation operation time of the process cartridge. This figure shows an example of a case where the image forming condition is set as a reference value of 200 xcexcm (line width) and the drum sensitivity and the developing property have a tendency to increase. Particularly, the fluctuating property in which the line width is abruptly fluctuated at an initial stage and then gradually approaches to 200 xcexcm as the accumulation operation time of the process cartridge is increased is shown.
Accordingly, an object of the present invention is to provide an image forming apparatus in which the fluctuation in the image forming property caused by an accumulation operation of a process cartridge or the image forming apparatus can be minimized.
The other objects and features of the present invention will be apparent from the following detailed explanation of the invention referring to the accompanying drawings.